Anatomic Variations of the Sacrum and Lumbosacral Junction: Clinical Implications for Orthopaedic and Spine Surgeons.

The sacrum is one of the most morphologically and functionally unique osseous structures of the human skeleton. As the keystone transmitting load from the axial skeleton through the sacroiliac joints down to the lower extremities, the sacrum's anatomic relationships with its surrounding neurovascular structures and its connection to the lumbar spine ("lumbosacral junction") are highly complex and important. Variations in the anatomy of the upper sacrum and the lumbosacral junction are relatively common and heterogeneous, as they range from sacral dysmorphism to partial and/or complete fusions of the lowest lumbar vertebra to the sacrum (lumbosacral transitional anatomy-Bertolotti syndrome). A fundamental understanding of such anatomic variations is critical for spine and orthopaedic surgeons, given that each anatomic variant can influence surgical planning, instrumentation strategies, and clinical outcomes of several common operations done to address sacropelvic injuries and lumbosacral spinal reconstructions. In this text, an overview of sacral anatomic development is accompanied by a detailed review of lumbosacral transitional anatomy and their associated imaging findings and classifications, as well as their clinical implications for orthopaedic and spine surgeons.